1. Field of the Invention
This invention relates to DC/DC converters used in AC/DC and DC/DC power supplies, and more particularly, in isolated multioutput power supplies with high output currents and also, in isolated power supplies with power factor correction capability.
2. Description of the Prior Art
Regulated DC/DC converters are commonly classified in one of three topologies: the buck; the boost; and the buck-boost. These converters comprise various arrangements of a switch, a diode, and a capacitor.
FIG. 1A illustrates a non-isolated, single switch buck-boost converter having a transistor switch SW.sub.1, an inductor L.sub.1, a diode D.sub.1, and a capacitor C.sub.1.
FIG. 1B illustrates a single output flyback converter having a transistor switch SW.sub.2, transformer T.sub.1, a catch diode D.sub.2, and a capacitor C.sub.2. The flyback converter is an isolated version of the buck-boost converter, as the transformer virtually is a coupled inductor. Since transformer primary winding has a leakage inductance, a snubber or clamp network is needed to prevent the transistor switch from a break-down.
The flyback converters are widely used in multioutput power supplies because of their simplicity. These converters do not have a transformer saturation problem. They are also distinguished by unity power factor capability when used in AC/DC power supplies. But these converters suffer significantly from current and voltage stresses that limit the output power.
FIG. 1C illustrates an interleaved flyback converter having two transistor switches SW.sub.3 and SW.sub.4, two transformers T.sub.2 and T.sub.3, two catch diodes D.sub.3 and D.sub.4, and a capacitor C.sub.3. The interleaved converter is a parallel combination of two converters of FIG. 1B controlled with a phase shift of 180.degree.. The interleaving reduces input and output current ripple.
The interleaved flyback converters may be also provided with a snubber or clamp network that absorbs voltage spikes generated by leakage inductance of the transformer primary when the switch turns off (see FIG. 1B).
In the interleaved topology of FIG. 1C, the energy of the voltage spikes may be also diminished by a secondary resonance in output LCD network (see U.S. Pat. No. 4,423,476 to Neumann).
FIG. 1D illustrates an active clamp technique in the interleaved flyback converter in accordance with U.S. Pat. No. 4,618,919 to Martin. This converter comprises transformers T.sub.4 and T.sub.5, output diodes D.sub.5 and D.sub.6, and switching means SW.sub.5 and SW.sub.6. A filter capacitor C.sub.4 is provided to absorb leakage inductance energy of transformer primaries when turning over the switching means SW.sub.5 and SW.sub.6. The capacitor C.sub.4 at the primary high voltage side of the converter is always coupled with the load R.sub.L on low voltage secondary side through one or both discharging transformers T.sub.4 and T.sub.5. As a result, a high equivalent output capacitance is achieved. However, a high percentage of converter energy still bounces between the transformers T.sub.4 and T.sub.5 and the filter capacitor C.sub.4.
A simplified single transistor active clamp circuit for an interleaved flyback converter is described by R. Watson, G.C. Hua, and F.C. Lee in the "Characterization of an active clamp topology for power factor correction application", IEEE Applied Power Electronic Conference Proceedings, 1994, pp. 412-418. Single transistor active clamp does not reduce the output current ripple. Also, active clamps used in combination with Zero Voltage Transition (ZVT) technique have been introduced in the flyback topology. An example of such technique is shown in U.S. Pat. No. 5,146,394 to Takuya Ishii et at. Unfortunately, the implementation of two active clamps and ZVT technique in the interleaved flyback converters requires a sophisticated control circuitry and many additional components.
FIG. 2 illustrates a two switch., buck derived push-pull converter comprising transistor switches SW.sub.7 and SW.sub.8, a transformer T.sub.6, diodes D.sub.7 and D.sub.8, a filter inductor L.sub.2, an output capacitor C.sub.5, and a primary side control circuit coupled with a current sensor CS. The Control Circuit drives gates G.sub.7 and G.sub.8 of the transistor switches with a phase shift of 180.degree.. The converter of FIG. 2 has the same number of components as the converter of FIG. 1C and may be considered as a conventional alternative of the latter operating in current continuous mode(CCM) for a single output application. The main disadvantage of converter of FIG. 1C is still a high output current ripple, that limits the output power. Besides that, the voltage across the transistor switches SW.sub.3 and SW.sub.4 when turning on, is higher, than the same in the converter of FIG. 2. It means higher turn-on losses caused by intrinsic capacitance of these switches.
A disadvantage of all flyback topologies of FIG. 1B, 1C, and 1D is that they feature unlinear transfer function. Another disadvantage that in most energy efficient CCM, the converters have right hand plane zero in its open loop frequency response (see K. Kit. Sum, "Switch Mode Power Conversion--Basic Theory and Design", Marcel Dekker Inc., 1984). When duty ratio of the switching transistors increases, duty ratio of output current pulses charging the output capacitor decreases, that is, average output current has a surge until the amplitude of these pulses will increase with some delay. It means that much more efforts are required to achieve good regulation and closed loop stability. Therefore, there is still a need for improved flyback converters that does not have the shortcomings of the prior art converters.